What is the point of animals like crane flies, where once they reach their adult phase their purpose is to lay eggs then die? They don't even have mouths (apparently) so. Their life cycle just seems so irrelevant like why would evolution do that? (Please no crane fly pics if u get to answering this question, I hate them very much D:) (sorry if this is phrased strangely)
Kind of an interesting question here, though you must be careful with words like ‘purpose’ when describing the way animals have evolved- there’s no purpose about it, it’s literally what randomly came together and worked.
The life cycle of the crane fly only seems confusing if you look at it from a human standpoint. Certainly it seems to us that the most proper life cycle includes a short nonreproductive juvenile period and a much longer reproductive-capable adult period. This, after all, is how most the lives of most vertebrates are structured. For example, a dog lives perhaps an average of twelve years, and only spends about six months of that time growing to sexual maturity.
And it does confer advantages from an evolutionary standpoint: having most of your life available to find mates seems like a pretty good way to maximize the number of offspring you produce. Here’s a really lazy timeline of that strategy, which in scientific terms is called an iteoparous lifestyle:
But there’s a danger in assuming that the juvenile period is wasted time, which it isn’t- otherwise it wouldn’t exist. Evolution rewards species that can successfully propagate themselves, and the timing of the nonreproductive period hinges on this. You see, there’s a slight problem with being ~READY TO BONE~ 24/7. Sexual organs, sexual secretions, and sexual behavior are all extraordinarily expensive. I’m not just talking about being sweaty and tired after a netflix and chill marathon. I’m talking about the biological costs incurred by producing eggs, sperm, secondary sex characteristics like giant antlers on deer and gaudy tails on peacocks, building nests for eggs, competing for opposite-sex attention and fighting off other suitors, and heck, even finding the dang object of your attraction. Think about how successful dating sites are, for goodness’ sake. In the US alone, about $80 million each year gets spent by horny people on dates.
Knowing how expensive all this can get, perhaps now it’s less surprising that some species want to make sure their offspring are as prepared as possible before they’re thrust into the Lust Pit. This may mean that they have proportionally longer juvenile periods than reproductive periods- however, when Fuck Time comes, they have a much better chance of finding a partner than you do on OkCupid because the entire species has synchronized their genitalia to develop at the same time. They may not even eat or sleep- they spend their last few weeks, days, or hours in a furious haze of lovemaking. Sometimes until they literally fall apart, in the case of the antechinus, a little marsupial that has such furious sex that he’ll lose all his hair and bleed internally (and then die). Which you wouldn’t expect when you see one:
This type of get-fucked-or-die-trying lifestyle is called semelparity, in contrast to our own iteroparity. Here’s another lazy timeline of that:
Semelparous animals sync up their breeding cycles to maximize their chances of finding a mate. This means it’d be pointlessly expensive to be reproductively primed during the off-season. Instead, they focus on preparation: growing as large and strong as they can so that when the time comes, they have the best chance possible. One of the best examples of this is the cicada, which is likely the longest-living insect- some species live up to 17 years. However, of those 17 years, only 2-4 weeks are spent as sexually mature adults. Emerging en masse after such a long absence not only makes it much easier to find a mate, it also overwhelms potential predators. Yes, cicadas are delicious, but you can only eat so many in two weeks compared to how many you could eat if they spent all seventeen years not buried deep underground.
Periodical cicadas are an extreme example, but many other animals have similar strategies. Calling something short-lived a “mayfly” refers to the fact that the sexually mature form is extraordinarily short-lived- in one species, it lives for less than five minutes. However, it’s often forgotten that this only refers to the adult form; the larvae will live possibly two years in rivers or streams.
It’s not just invertebrates that practice extreme semelparity. I already mentioned the little antechinus- the males of that species, by the way, live less than a year, while the females live for two years and generally die after weaning their first litter. Pacific salmon are another familiar semelparous species, which spend up to five years in the ocean before returning to freshwater to spawn and die within the span of a few days.
Perhaps the most extreme example of a semelparous vertebrate that I know of is Labord’s chameleon. The eggs of this species take roughly 9 months to incubate before hatching. After hatching, the juveniles reach sexual maturity at about two months old- and die another two months later. That’s right: this species of chameleon spends more time in an egg than it does in the outside world. Not only that, but because the mating takes place seasonally, there are long periods of time in which no adult individuals of the species exist. All of them are encased in eggs- silently growing, and preparing for the pinnacle of their lives: the Great Fuckening.
Dobson, F. S. (2013). Live fast, die young, and win the sperm competition. Proceedings of the National Academy of Sciences, 110(44), 17610-17611.
Karsten, K. B., Andriamandimbiarisoa, L. N., Fox, S. F., & Raxworthy, C. J. (2008). A unique life history among tetrapods: an annual chameleon living mostly as an egg. Proceedings of the National Academy of Sciences, 105(26), 8980-8984.
Koenig, W. D., & Liebhold, A. M. (2013). Avian predation pressure as a potential driver of periodical cicada cycle length. The American Naturalist, 181(1), 145-149.
Williams, K. S., Smith, K. G., & Stephen, F. M. (1993). Emergence of 13-Yr periodical cicadas (Cicadidae: Magicicada): phenology, mortality, and predators satiation. Ecology, 1143-1152.
Young, T. P. (2010). Semelparity and iteroparity. Nat Educ Knowl, 3(2).
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